On an industrial scale, organic isocyanates are generally produced by reacting the corresponding amines with phosgene. Due to the toxicity of phosgene, attempts have long been made to find a commercially workable method of synthesizing organic isocyanates in which phosgene need not be used. One such synthesis comprises reacting organic nitro compounds with carbon monoxide and organic hydroxyl compounds to form the corresponding urethanes and subsequently splitting the urethanes thus formed into isocyanate and compounds containing hydroxyl groups. The urethane obtained as an intermediate product may even be modified before splitting. Thus, it possible, for example, to initially react the phenyl urethane obtainable from nitrobenzene, carbon monoxide and ethanol with formaldehyde to form the bis-urethane of 4,4'-diisocyanatodiphenyl methane and thereafter to convert the intermediate product thus obtained into 4,4'-diisocyanatodiphenyl methane by splitting off the ethanol.
The splitting of urethanes into the corresponding isocyanates and compounds containing hydroxyl groups is described, inter alia, in German Offenlegungsschrift No. 2,421,503.
Two main types of catalyst are described in the patent literature for the production of urethanes. Thus, German Offenlegungsschriften Nos. 2,343,826; 2,614,101 and 2,623,694 describe the reaction of organic nitro compounds with carbon monoxide and alcohols in the presence of selenium or selenium compounds to form urethanes. High urethane yields are obtained both with mono- and also with di-nitro compounds. The selenium compounds, particularly the organic selenium compounds formed as an intermediate stage during the reaction, and hydrogen selenide, are extremely toxic and must be quantitatively removed during working up, for example by a chemical reaction. This results in the need for an elaborate, chemical working-up stage which will adversely affect the economy of the process.
German Offenlegungsschriften Nos. 1,568,044 and 2,603,574, describe the use of noble metals, particularly palladium, in the presence of Lewis acids as catalysts. Anhydrous iron(III)chloride is disclosed as being a particularly effective Lewis acid. Although high urethane yields, based on the nitro compound used, are obtained using these catalysts, the yields based on the hydroxy compound used are unsatisfactory. Thus, where ethanol is used as the hydroxy component, large fractions of diethyl ether are obtained, the diethyl ether being formed because of the acid properties of the Lewis acid. At the same time, corrosion of the fine steel autoclaves used as reaction vessels is observed in cases where these noble metal/Lewis acid catalysts are used. Although this corrosion can largely be avoided by the addition of organic bases, such as pyridine (see, e.g., German Offenlegungsschrift No. 2,604,574), ether formation is still unreasonably high in the presence of these catalyst systems. Another disadvantage of these catalyst systems is that they can only be re-used to a limited degree. This is because the Lewis acids used are not sufficiently stable in the presence of the hydroxy compounds used.